Fibers are typically classified according to their diameter. Monofilament fibers are generally defined as having an individual fiber diameter greater than about 15 denier, usually greater than about 30 denier per filament. Fine denier fibers generally refer to fibers having a diameter less than about 15 denier per filament. Microdenier fibers are generally defined as fibers having less than 100 microns in diameter. Fibers can also be classified by the process by which they are made, such as monofilament, continuous wound fine filament, staple or short cut fiber, spun bond, and melt blown fibers.
Fibers with excellent elasticity are needed to manufacture a variety of fabrics which are used, in turn, to manufacture a myriad of durable articles, such as sport apparel and furniture upholstery. Elasticity is a performance attribute, and it is one measure of the ability of a fabric to conform to the body of a wearer or to the frame of an item. Preferably, the fabric will maintain its conforming fit during repeated use, extensions and retractions at body and other elevated temperatures (such as those experienced during the washing and drying of the fabric).
Fibers are typically characterized as elastic if they have a high percent elastic recovery (that is, a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties: (i) a low percent permanent set, (ii) a low stress or load at strain, and (iii) a low percent stress or load relaxation. In other words, elastic materials are characterized as having the following properties (i) a low stress or load requirement to stretch the material, (ii) no or low relaxing of the stress or unloading once the material is stretched, and (iii) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Spandex is a segmented polyurethane elastic material known to exhibit nearly ideal elastic properties. However, spandex is cost prohibitive for many applications. Also, spandex exhibits poor environmental resistance to ozone, chlorine and high temperature, especially in the presence of moisture. Such properties, particularly the lack of resistance to chlorine, causes spandex to pose distinct disadvantages in apparel applications, such as swimwear and in white garments that are desirably laundered in the presence of chlorine bleach.
A variety of fibers and fabrics have been made from thermoplastics, such as polypropylene, highly branched low density polyethylene (LDPE) made typically in a high pressure polymerization process, linear heterogeneously branched polyethylene (e.g., linear low density polyethylene made using Ziegler catalysis), blends of polypropylene and linear heterogeneously branched polyethylene, blends of linear heterogeneously branched polyethylene, and ethylene/vinyl alcohol copolymers.
In spite of the advances made in the art, there is a continuing need for polyolefin-based elastic fibers which are soft and yielding to body movement. Preferably, such fibers would have relatively high elastic recovery and could be made at a relatively high throughput. Moreover, it would be desirable to form fibers which do not require cumbersome processing steps but still provide soft, comfortable fabrics which are not tacky.